What is the X in X-mas?
histogenesis his·to·gen·e·sis (hĭs'tō-jěn'ĭ-sĭs)
The formation and development of the tissues of the body.
series of organized, integrated processes by which cells of the primary germ layers of an embryo differentiate and assume the characteristics of the tissues into which they will develop. Although the final form of the cells that compose a tissue may not be evident until the organ itself is well along in development, distinctive biochemical reactions, which are the signatures of histogenesis, can be detected much earlier.